thunderbolt: Prevent crash when ICM firmware is not running

On Lenovo ThinkPad Yoga 370 (and possibly some other Lenovo models as
well) the Thunderbolt host controller sometimes comes up in such way
that the ICM firmware is not running properly. This is most likely an
issue in BIOS/firmware but as side-effect driver crashes the kernel due
to NULL pointer dereference:

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000980
  IP: pci_write_config_dword+0x5/0x20
  Call Trace:
   pcie2cio_write+0x3b/0x70 [thunderbolt]
   icm_driver_ready+0x168/0x260 [thunderbolt]
   ? tb_ctl_start+0x50/0x70 [thunderbolt]
   tb_domain_add+0x73/0xf0 [thunderbolt]
   nhi_probe+0x182/0x300 [thunderbolt]
   local_pci_probe+0x42/0xa0
   ? pci_match_device+0xd9/0x100
   pci_device_probe+0x146/0x1b0
   driver_probe_device+0x315/0x480
   ...

Instead of crashing update the driver to bail out gracefully if we
encounter such situation.

Fixes: f67cf491 ("thunderbolt: Add support for Internal Connection Manager (ICM)")
Reported-by: Jordan Glover <Golden_Miller83@protonmail.ch>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Yehezkel Bernat <yehezkel.bernat@intel.com>
Cc: stable@vger.kernel.org